2000PASP..112.1595H

We present an analysis of ground-based optical photometry and spectroscopy and Rossi X-Ray Timing Explorer X-ray observations of the old novae DI Lacertae and V841 Ophiuchi. Our optical photometry data (obtained with the automated photometry telescope RoboScope) comprise an almost decade-long light curve for each star, while the contemporaneous spectroscopy and X-ray observations repeatedly sampled each nova during separate intervals of ~45-55 days in length. The long-term optical light curves of both novae reveal quasi-periodic variability on typical timescales of ∼30-50 days with amplitudes of ΔV∼0.4-0.8 mag. V841 Oph also displays a long-term, sinusoidal modulation of its optical light on a timescale of 3.5-5 yr. The optical spectra of these novae display quite different characteristics from each other, with DI Lac showing narrow Balmer emission cores situated in broad absorption troughs while V841 Oph exhibits strong single-peaked Balmer, He I, and He II emission lines. We find little change between spectra obtained during different optical brightness states. The X-ray count rates for both novae were very low (≲1.5 counts.s^–1), and there was no reliable correlation between X-ray and optical brightness. The combined X-ray spectrum of DI Lac is best fit by a bremsstrahlung emission model (with kT∼4 keV and N_H<1.8x10²² cm^–3); the X-ray spectrum of V841 Oph is too weak to allow model fitting. We discuss the possible origin of variability in these old novae in terms of magnetic activity on the secondary star, dwarf nova type disk instabilities, and the ``hibernation'' scenario for cataclysmic variable stars.